Global Radiative-Convective Equilibrium in the Community Atmosphere Model: Understanding Model Sensitivities

Abstract:

In our continued effort to understand the climate system and improve its representation in general circulation models (GCMs) it is crucial to develop new methods to evaluate these models. This is certainly true as the GCM community advances towards high horizontal resolutions (i.e., grid spacing less than 0.5 degrees), which will require interpreting and improving the performance of many model components. Of specific interest is the role of convective parameterizations at these spatial scales and its impact on tropical dynamics and precipitation processes. Idealized, or reduced complexity, frameworks can be used to investigate how model assumptions impact behavior across scales. A simplified global radiative-convective equilibrium (RCE) configuration is proposed to explore the implication of horizontal resolution on equilibrium climate. RCE is the statistical equilibrium in which the radiative cooling of the atmosphere is balanced by heating due to convection and has had a fundamental place in our understanding of the Earth system.

In this work, the National Center for Atmospheric Research’s Community Atmosphere Model 5 (CAM5) is configured in RCE to better understand tropical climate and extremes. The RCE setup consists of an ocean-covered earth with diurnally varying, spatially uniform insolation and no rotation effects. CAM5 is run with the spectral element dynamics package at two horizontal resolutions: a standard resolution of approximately 1 degree grid spacing and a high-resolution of approximately 0.25 degree grid spacing. Surface temperature effects are considered by comparing simulations using fixed, uniform sea surface temperature to simulations using an interactive slab ocean model. The various CAM5 configurations provide useful insights into the simulation of tropical climate at the high-resolution, as well as the model’s ability to simulate extreme precipitation events. In particular, the manner in which convection organizes is shown to be dependent on model resolution, as evident by differences in cloud structure, circulation, and precipitation intensity. The use of the interactive slab ocean also impacts convective characteristics, by increasing convective temporal variability and intensity.